Bestflex/Nestaflex left side rail drop in prevents falling product. Use on troublesome corner or entire conveyor. Fits any Bestflex or Nestaflex with square frame opening from 1 1/4" to 1 1/2" inside measurement. Pop off plastic cap and drop in. Use one per frame per side, sold individually. Rails slip over each other from frame to frame so conveyor can expand and retract.Suggest having end bracket part number: 403661 at end of line to hold rail in place. Picture shown is shipping application. Use arrow in pic to figure out which side you need for ordering. 4" x 43" measurements.